Mortar concrete is subjected to a hydration reaction upon addition of water thereto and its consistency drops with the lapse of time. Such a lowering of consistency gives rise to some trouble in placing or compacting of concrete, although the degree of such a lowering may be depending upon the kinds of cements, aggregates or admixtures, the mixing conditions and the post-treatment conditions.
Recently wide acceptance of air-entraining agents or cement dispersants plays an important role in improvements in the various physical properties of mortar cement. However, such air-entraining agents and/or cement dispersants have little or no affect on the prevention of a lowering of consistency with the lapse of time and rather have a tendency to promote it.
Among the cement dispersants a so-called high-powered dispersant which excels especially in the dispersing effect and may be used in considerable amounts with causing any adverse influence such as delays in setting or excess entrainment of air is being widely used for the production of mortar concrete demanded to be of high strength, inter alia, secondary products at a factory. It is noted, however, that the addition of such a high-powered dispersant, for instance a high condensation product of naphthalene sulfonate and formaldehyde, a co-condensation product of naphthalene sulfonate and lignin sulfonate formaldehyde or sulfonate product of a melamine formaldehyde condensation product to mortar concrete results in a marked lowering of consistency with the lapse of time as compared with that having no admixture.
This lowering of consistency with the lapse of time presents many problems including:
1. Limited period of time during which the ready-mixed mortar concrete can be transported; PA1 2. Variations in the quality of the placed ready-mixed mortar concrete which are caused by changes in consistency due to changes in the preservation time at a placing region; PA1 3. Limitations placed on the transportation distance of concrete by pumping and clogging or jamming of piping; PA1 4. Unsatisfactory compacting caused in case of using centrifugal molding.
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In order to solve these problems such various methods as mentioned below have heretofore been proposed or actually carried out; however, no satisfactory results are still obtained.
Among these methods, there is a method for obtaining a somewhat large increase in consistency at the time of mixing by increasing the amount of water added per unit, thereby compensating for a lowering of consistency during the preservation of concrete. In this method, however, the water to cement ratio increases with decreases in strength and durability. In addition, there is a fear that segregation or clogging of a pump may be caused. Since the centrifugal molding permits an given amount of water to be squeezed out the concrete, the addition of an increased amount of water seemingly gives rise to no difficulties, but a considerably degree of segregation possibly takes place, thus leading to losses of strength.
In order to avoid this defect, an additional step of increasing the weight of cement per unit in proportion to increases in the quantity of water per unit may be provided; although this step is useful for the prevention of a lowering of strength, however, it results in not only an economical disadvantage but also a great degree of shrinkage and hence cracking of the cured mortar cement.
In a word, the aforesaid method for adjusting the consistency of the concrete upon mixing to above a given value by increasing the amount of water per unit offers no satisfactory solution for reducing an essential lowering of consistency with the lapse of time.
On the other hand, another conventional method which is old but frequently used in recent years is a so-called "delayed addition process for cement dispersants". More specifically, this process involves the addition of a cement dispersant, which must substantially be added at the starting point of mixing, after several tens seconds to a few minutes of the addition of water to increase the consistency of the resultant concrete, and hence brings about improvements in delaying the rate of setting and prevents a lowering of the consistency with the subsequent lapse of time. Compared with the method merely permitting the quantity of water to be increased, this process is said to be an efficient way; however, no satisfactory results are still obtained for the effect of reducing a lowering of consistency.
Furthermore, this process has such disadvantage that mixing is difficult till the addition of dispersants due to the absence of any dispersant at the time of initial mixing. In particular, its practical use is impossible when use is made of a concrete mixture which has a small water to cement ratio and must therefore contain the high-powered cement dispersant. In order to overcome this disadvantage, a so-called "division addition method" is proposed, which comprises introducing a part of the dispersant at the starting point of mixing and the remainder in the course of mixing. In any event, this process is no more than a variation of the delayed addition process and is considered to have a more unfavorable effect on the prevention of a lowering of consistency with the lapse of time than do the delayed addition process.
Recently, the spotlight of attention is focused upon a so-called "in-situ addition process" which is designed to add in-situ the high-powered dispersant to mortar concrete having a reduced consistency to thereby restore the consistency to the original value. Upon restoration, however, the life time of the concrete is at most 25 to 30 minutes as the consistency of the concrete incorporated with the high-powered dispersant drops sharply. In order to extend the life time, it may be possible to add the high-powered dispersant in several portions. However, this method is troblesome to operate and difficult to control so that material segregation may occur depending upon the mixing conditions. This results in the set mortar concrete of inferior quality being produced.
As will be evident from the foregoing, while the cement dispersants have heretofore played a significant role in improvements in the various properties of the mortar concrete, they have a tendency toward promoting a lowering of consistency with the lapse of time. In order to cope with this problem, various attempts have been proposed or reduced to practical use. Nonetheless, it is found that they offer neither essential solution nor satisfactory result.
Under the situation being like this, there is a strong demand for the development of more efficient admixtures and methods for permitting prevention of a lowering the consistency of mortar concrete.